To estimate the whole-body oxidation rates of phenylalanine and tyrosine and body aromatic amino acid balance, by comparing 13C-phenylalanine and 13C-tyrosine as tracers, and measuring13C-tyrpsine enrichment. This may give us a better estimate of preformed tyrosine oxidation, plus tyrosine formed from phenylalanine that mixes with the free tyrosine pool, i.e. non-channelled phe-tyr formation.